1. Field of the Invention
This invention relates in general to optical components, and in particular to a polarization-independent air-path (free-space) isolator utilizing a beam aperture and/or an offset beam aperture method.
2. Description of Related Art
Optical air-path isolators are one of the most important passive components to be used in laboratories, testing and related technologies. The function of an optical isolater is to allow a light beam to pass through it in one direction, for example, the foward direction only (like a one-way traffic). The isolator dramatically suppresses the light propagating in the backward direction.
The current polarization-independent air-path (free-space) isolator is based on the Faraday effect, which is one of the most commonly used nonreciprocal effects. A stage of the art air-path (free-space) isolator basically consists of a Faraday single crystal (for example, Garnet), a hollow magnet and two birefringent crystals (for example, LiNbO.sub.3). The Faraday crystal is placed inside the hollow magnet and between these two birefringent crystals. Collimated or nearly-collimated beams propagating through the air are usually required as the incident beams for the isolator. That is why the isolator is called an air-path or free space one.
The forward-going beam from the input side, upon impinging on the first birefringent crystal, becomes two spatially separated beams (the ordinary and extraordinary beams with orthogonal polarization vectors, that is, the O-beam and the E-beam) due to the birefringence. After passing the Faraday crystal and the second birefringent crystal, these two beams are paralleled to each other and therefore completely get through the small aperture at the output end. However, the O-beam and the E-beam of the backward-going beam emerging from the first birefringent crystal, become divergent due to the nonreciprocal Faraday effect. The divergent beam cannot pass through the small aperture at the input end. Thus, the function of an isolator (to suppress the backward-going beam) is realized. The structure of such an isolator is quite complicated, and accurate optical alignments between these parts are required. The high cost of the components and labors leads to a relatively expensive air-path isolator.
There is therefore an urgent need for another type of air-path isolator that significantly reduces the number of components, resulting in a simpler structure, a faster assembly time, lower cost, higher reliability and improved performance.